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Figure 3: Input current when powered by an LET75 at 120 VAC without a dimmer. This waveform is taken with a 40 µs time base and clearly shows the extra load added by the circuit comprised of Q3 and Q4. This load is removed after the first 80µs on every half-cycle of the AC waveform. Brightness increases, brightness decreases – this is not the consistent operation demanded of most applications today. It is possible, however, to keep the brightness of an MR16 LED lamp constant when the line varies around nominal input voltage. However, LED MR16 lamps are not resistive loads, which is what the electronic transformers require. Therefore, the loading behavior of the LED MR16 lamp needs to be adjusted so it can draw the power required to provide the desired light output and keep the electronic transformer operational.
The correct MR16 LED Driver/Transformer is an investment for your MR16 LED bulb ensuring maximum lifespan and perfect operation. With LED lighting power supplies you are guaranteed our MR16 LED drivers are designed to perform with the latest LED technology. Figure 7: LED current waveform when powered by an LET75 with a trailing-edge dimmer at 120 VAC. The dimmer is set at maximum light output. There is another way to solve the problem: reduce the boost inductor to 10 µH, operate at high switching frequencies, and remove the additional load. The high switching frequencies will cause higher switching losses, but would not need the additional load. Both the above mentioned methods are proprietary to Maxim Integrated. By using an HB LED driver optimized for MR16 and other 12VAC lamps, you can make MR16 LEDs compatible with electronic transformers. The LED driver demonstrated here was the MAX16840. One should note, however, that the performance of each, distinct electronic transformer and dimmer combination must be tested.
Brightness increases, brightness decreases—this is not the consistent operation demanded of most applications today. It is possible, however, to keep the brightness of an MR16 LED lamp constant when the line varies around nominal input voltage. But LED MR16 lamps are not resistive loads, which is what the electronic transformers require. Therefore, the loading behavior of the LED MR16 lamp needs to be adjusted so it can draw the power required to provide the desired light output and keep the electronic transformer operational. The driver circuitry of the MR16 LED lamp can be adjusted so that it draws a constant current load from the output of the electronic transformer. No capacitance can be added to the electronic transformer's output, since this can prevent the MR16 LED lamp from acting as a constant current load. Moreover, the current drawn by the MR16 LED lamp needs to ramp up to the programmed current at a very fast rate. Specifically, it needs to jump to the programmed value within 3µs or 4µs. If it ramps up slower than this, then the electronic transformer may stop switching.
Performance data are shown here for a 6W boost, MR16 LED lamp when powered by different electronic transformers. The boost MR16 was tested with 7 LEDs on the output. Tables 1 and 2 summarize performance with different transformers. Performance data are shown here for a 6 W boost, MR16 LED lamp when powered by different electronic transformers. The boost MR16 was tested with seven LEDs on the output. Tables 1 and 2 summarize performance with different transformers.The transformer you need for strip lighting depends on multiple factors, such as the length of strip, watts per chip and total wattage. LED strip lighting comes with either small LED chips or larger LED chips, as well as a variety of sizes ranging from 1m to 5m. Small LED strip light chips are marked “3528” and measure 3.5mm x 2.8mm, while large LED strip light chips are marked “5050” and measure 5mm x 5mm. Larger 5050 chips are brighter per metre than small 3528 chips. To calculate the total wattage of the LED strip you wish to power you have to do one of these simple sums: • 60 small LED chips: calculate 5 watts per metre • 120 small LED chips: calculate 10 watts per metre • 60 large LED chips: calculate 15 watts per metre You then need to add another 20% to avoid potential overload on the transformer.
By using an HB LED driver optimized for MR16 and other 12 VAC lamps, you can make MR16 LEDs compatible with electronic transformers. The LED driver demonstrated here was the MAX16840. One should note, however, that the performance of each, distinct electronic transformer and dimmer combination must be tested. If you’re switching to LED MR16s from halogen MR16s, then LED drivers are pretty important. They’re used to power low voltage (12v) LED bulbs and strip lighting, providing a constant current to make sure they work properly. They’re mainly used for strip lighting, and MR16, MR11 and G4 spotlight bulbs. In some cases your existing (normal) 12v transformer will work. However, this can reduce the lifespan of the bulb. You may have bought or are thinking about buying some LED strip lighting or MR16 fittings or spotlights for your home. Either way you’re here to find out the whats and whys about LED drivers (or transformers). This handy guide will help you make an informed decision and give you the fundamentals you need when buying a driver. Why do I need an LED driver? To solve this problem, this circuit design has an additional load consisting of R18, D7, C14, Q4, D8, R17, R11, R13, and Q3. This circuit adds 5Ω to the electronic transformer, but only for approximately 80µs to 90µs per half-cycle of the rectified AC sine wave, which typically has a frequency of 100Hz/120Hz. The load is removed as soon as the current in the inductor has ramped up sufficiently high to keep the transformer operational. The power dissipated by this additional load is small.MR16 halogen lamps normally operate from a low-voltage AC source typically generated by a low-frequency AC transformer or by a high-frequency electronic transformer. In most MR16 applications, the high-voltage AC provided by electric power companies is converted to a low-voltage AC by a high-frequency electronic transformer or by a low-frequency magnetic transformer. A high-frequency electronic transformer has a primary winding that connects directly to the 120VAC/230VAC. It uses high switching frequencies to provide the low voltage (12VAC) which is applied to the MR16 halogen lamp. Figure 7. LED current waveform when powered by an LET75 with a trailing-edge dimmer at 120VAC. The dimmer is set at maximum light output. A similar version of this article appeared on Display Plus, July 7, 2012 and in German in Elektronikpraxis, October 1, 2012. Introduction
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